This invention relates generally to the construction of gate valves of the fabricated type and to methods for their manufacture.
Gate valves when made in the larger sizes frequently have bodies fabricated by welding from steel mill shapes such as plates, channels or angles. The main body part, which connects with associated piping and carries the sealing assemblies, must be made sufficiently strong to withstand line pressures and also stresses applied by the connecting piping. When the valve is of the through port type, in which the gate has a port which is aligned with the flow passages of the valve body for open valve position, there are upper and lower extensions of the body which are either integral with the main body part, or separately fabricated and secured to the main body part. Such extensions accommodate movement of the valve gate between full open and closed positions. When the valve is not of the through port type, and employs a relatively short gate, only an upper body extension is provided, which has bonnet means serving to carry a manual or power operator. Whether the body has one or two body extensions, it is evident that these extensions must be constructed in such a manner as to withstand the line pressure as a pressure vessel. When one or both body extensions have the same configuration as the main body part, as for example, a box that is rectangular in section, it must be made with relatively heavy end walls or with reinforcing ribbing associated with the end walls to withstand internal line pressure, thus increasing weight and cost of manufacture. As shown in U.S. Pat. No. 3,372,903, the upper body extension may be a separate structure that is dome-shaped or cylindrical to more nearly approximate a theoretical ideal pressure vessel. This makes possible thinner walls and a saving in weight. However, such dome-shaped or cylindrical extensions are relatively expensive to manufacture and to mount upon the main body part.
In some instances body extensions of large sized valves have been pressure formed during manufacture to approximate a spherical pressure vessel. Such body extensions may have end walls that correspond to segments of a sphere, with curved peripheral edges welded to curved side walls. While such extensions may provide desirable pressure resisting requirements for a given weight, the width as measured horizontally is substantially greater than the width of the main part of the body, whereby a valve making use of such an extension has a bulky configuration which may be undesirable in many installations. In addition, the fabrication of such extensions requires special welding operations which differ from the relatively simple welding operations normally used in the fabrication of gate valves.